In Long Term Evolution (LTE), a small cell having a small cell radius is defined as well as a macro cell having a large cell radius. In order to increase communication speed in a small area while taking advantage of a characteristic of covering a smaller area than a macro cell covering a broad area, a plurality of small cells are formed in a high-load region such as a downtown and a commercial building. In addition, in order to cover an indoor area within a macro cell where radio waves are hard to reach, a small cell is also formed indoors. Because of such a use application of a small cell, a macro cell and a plurality of small cells are often in an adjacent relationship with respect to each other. However, a plurality of small cells are not always in an adjacent relationship with respect to each other.
FIG. 1 illustrates an example of cell arrangement in an LTE wireless communication system.
In FIG. 1, Cell X that is a macro cell and Cells Y and Z that are small cells are in an adjacent relationship with respect to each other. However, Cell Y and Cell Z are not in an adjacent relationship with respect to each other.
Incidentally, in LTE, a Physical Cell Identity (PCI) is used as a cell identifier for identifying a cell. It is assumed herein that the term “cell identifier” refers to the PCI. The PCI is a cell identifier locally used for user equipment (UE) to identify a cell on a wireless section, and there are five hundred and four PCI values that are repeatedly used in LTE.
Thus, as illustrated in FIG. 1, there is a case in which Cells Y and Z adjacent to Cell X may have duplicate PCI values (=84). Such PCI value duplication is referred to as PCI confusion.
Herein, Cell X and Cells Y and Z are in an adjacent relationship with respect to each other. Thus, there is a case in which small cell base stations (hereinafter, referred to as Small cell base stations y and z) respectively forming Cells Y and Z can acquire neighbor information indicating neighboring cells adjacent to Cell X from a macro cell base station (hereinafter, referred to as Macro cell base station x) forming Cell X via a connection line or the like (e.g., X2 Interface). When Small cell base stations y and z can acquire neighbor information from Macro cell base station x, Small cell base stations y and z can determine that, on the basis of the neighbor information, PCI Confusion is occurring, in which Cells Y and Z have PCI values duplicate to each other. In this case, when any of Small cell base stations y and z autonomously re-selects a PCI value, the PCI confusion is eliminated.
However, there is a case in which Small cell base stations y and z cannot directly acquire neighbor information from Macro cell base station x via a connection line or the like. In addition, there is a case in which, even when Small cell base stations y and z can acquire neighbor information from Macro cell base station x, some of neighboring cells adjacent to Cell X cannot be recognized due to insufficiency of the neighbor information. In addition, since Cell Y and Cell Z are not in an adjacent relationship with respect to each other, Small cell base stations y and z cannot acquire neighbor information from the other side's small cell base station.
Accordingly, Small cell base stations y and z cannot determine that a PCI value of the own cell has caused PCI confusion in which the PCI value of the own cell is duplicated with a PCI value of the other cell in any of the cases described above. Small cell base stations y and z lose an opportunity to autonomously re-select a PCI value, and the PCI confusion is not eliminated.
Herein, think about a case in which a UE moves from Cell X to Cell Y while PCI confusion is occurring, in which PCI values of Cells Y and Z are duplicated to each other.
In this case, the UE transmits, to Macro cell base station x forming Cell X, a Measurement Report message including a PCI value “84” of Cell Y that is a handover destination candidate.
However, the PCI confusion state does not allow a value of an ECGI (E-UTRAN Cell Global Identifier. E-UTRAN: Evolved Universal Terrestrial Radio Access Network) corresponding to a PCI value to be uniquely determined. Herein, the ECGI refers to a cell identifier for uniquely identifying a cell throughout a communication network, unlike the PCI.
Thus, there is a case in which Macro cell base station x may request for handover not to Cell Y that is a handover destination candidate, but to wrong Cell Z having the same PCI value as that of Cell Y. In this case, there is a problem that handover fails and this results in a decrease in a handover success rate.
Examples of a technique for solving the problem include a method described in PTL 1. In the method described in PTL 1, while assuming a case in which there are a plurality of small cells using the same PCI value in a macro cell, UE adds, to a measurement report, a PCI value and a CGI value of a handover destination candidate cell.